1. Field of the Invention
The present invention relates generally to a packet data service system, and in particular, to a wireless packet data service system in a wireless mobile environment.
2. Description of the Related Art
Mobile communications systems are being developed to be high-speed, high-quality wireless packet data communications systems to additionally provide data service and multimedia service beyond voice-oriented service that was available at an early development stage.
The 3rd Generation Partnership Project (3GPP) standardization of High Speed Downlink Packet Access (HSDPA) and the 3GPP2 standardization of 1×Evolution for Data and Voice (EV-DV) in Release 5 are good examples of the efforts to find a solution to wireless data packet transmission service with high quality at a high speed of 2 Mbps or above in a 3G mobile communication system. 4th Generation (4G) mobile communication systems aim to provide higher-speed, higher-quality multimedia service. Enhanced Uplink Dedicated Channel (EUDCH) under discussion in Release 6 is also an example of one of the efforts to transmit high-speed, high-quality wireless data packets on the uplink.
Obstacles to high-speed, high-quality data service in wireless communications are caused mainly by a radio channel environment. The radio channel environment frequently changes due to signal power changes by fading as well as white noise, shadowing, Doppler effects caused by user mobility and frequent mobile velocity changes, and interference from other users and multipath signals.
To provide high-speed wireless packet data service, therefore, other advanced technologies than those provided in the existing 2nd Generation (2G) or 3G mobile communication systems are needed for increasing the adaptability to channel changes. Although fast power control used in the legacy systems increases the adaptability to channel changes, the 3GPP and the 3GPP2, which are working on the standardization of high-speed packet data transmission systems, commonly address Adaptive Modulation and Coding (AMC) and Hybrid Automatic Repeat reQuest (HARQ).
The AMC adaptively selects a modulation scheme and a coding rate according to the downlink channel environment. The downlink channel environment is evaluated based on feedback information about a Signal-to-Noise Ratio (SNR) measurement received from a User Equipment (UE), and the modulation scheme and the coding rate are determined according to the downlink channel environment.
In an AMC system, a high-order modulation scheme such as 16-ary Quadrature Amplitude Modulation (16QAM) or 64QAM and a high coding rate such as 3/4 are selected for a good channel environment in which a UE near to a Node B is usually situated. For a UE at a cell boundary, a low-order modulation scheme like Quadrature Phase Shift Keying (QPSK) or 8-ary Phase Shift Keying (8PSK) and a low coding rate like 1/2 are used. This AMC technology reduces interference and improves system performance on the whole, compared to the conventional fast power control.
The HARQ is a link control scheme for the case where, in the presence of errors in an initially transmitted packet, packet retransmission is requested to compensate for the error packet. The HARQ is broken up into Chase Combining (CC), Full Incremental Redundancy (FIR), and Partial Incremental Redundancy (PIR).
In the CC, the same packet as initially transmitted is retransmitted. A receiver combines the retransmitted packet with the initially transmitted packet buffered in a reception buffer, thereby increasing the reliability of coded bits input to a decoder and thus achieving an overall system performance gain. Combining the same two packets is equivalent to iterative coding in effect. Thus, a performance gain of about 3 dB is achieved on the average.
The FIR improves decoder performance at the receiver by transmitting a retransmission packet having only redundant bits generated from a channel encoder, rather than retransmitting the same packet. As the decoder uses both the initial transmission information and the new redundant bits, the resulting decrease in coding rate increases the decoder performance. It is well known in coding theory that a higher performance gain is obtained by a low coding rate than by iterative coding. Only in terms of performance gain, the FIR outperforms the CC.
In contrast to the FIR, the PIR transmits packet data with information bits and new redundant bits at a retransmission. At decoding, the information bits are combined with initially transmission information bits, thereby achieving the same effects as the CC, and the use of the redundant bits results in the effects of the FIR. Since the PIR uses a higher coding rate than the FIR, it performs between the FIR and the CC. However, since many considerations including system complexity such as a buffer size at the receiver and signaling, as well as performance must be taken into consideration when selecting an HARQ technique, it is not easy to determine one of the above HARQ techniques.
While the AMC and the HARQ are independent technologies for increasing adaptability to link changes, the use of them in combination can remarkably improve system performance. Once a modulation scheme and a coding rate are adaptively determined according to the downlink channel condition by the AMC, packet data is correspondingly transmitted. If the receiver fails to decode the packet data, it requests a retransmission. The Node B retransmits predetermined packet data by a predetermined HARQ scheme in response to the retransmission request.
While the above-described schemes are applicable to HSDPA, 1×EV-DV, and EUDCH, they will be described herein in the context of channels used for HSDPA. FIG. 1 is a diagram illustrating the timing relationship between two channels adopted to support HSDPA, a High Speed Shared Control CHannel (HS-SCCH) and a High-Speed Physical Downlink Shared CHannel (HS-PDSCH). The HS-PDSCH is a transport channel for delivering packet data from a transmitter in a Node B to a receiver in a UE, and the HS-SCCH carries control information for supporting the HS-PDSCH.
Referring to FIG. 1, the UE demodulates the HS-SCCH for a time period (HS-PDSCH) being two slots before the HS-PDSCH to acquire control information necessary for demodulation of the HS-PDSCH. In FIG. 1, Tslot represents a time slot which is 2,560 chips. A High-Speed Downlink Shared CHannel (HS-DSCH) is a downlink transport channel that carries high-speed downlink packet data. It is composed of one or more HS-PDSCHs. The HS-PDSCH is a physical channel that delivers the HS-DSCH.
The HS-SCCH is divided into two parts each delivering control information as illustrated in Table 1 below. Numerals in brackets indicate the number of information bits.
TABLE 1Part 1Part 2Channelization Code Set (7)Transport Block Size (6)Modulation Scheme (1)HARQ Process ID (3)Redundancy and Constellation Version (3)New Data Indicator (1)UE Identification (16)
The UE has to monitor up to at most four HS-SCCHs and selects one of them intended for the UE. Thus, if the UE determines that control information has been transmitted for the UE after demodulating at most four the HS-SCCHs, the UE has to demodulate the HS-PDSCH. If the HS-PDSCH is not intended for the UE, the UE demodulates the next HS-SCCHs.
The above control information delivered on the HS-SCCH illustrated in Table 1 will be described in more detail.
The Channelization Code Set (CCS) indicates the number of channelization codes used for the HS-PDSCH. The CCS is 7 bits and provides the number and types of codes with which the UE performs despreading. Up to 15 channelization codes are available for the HS-PDSCH according to the current standards.
In HSDPA, QPSK and 16QAM are available and the Modulation Scheme (MS) indicates which is to be used. Since the CCS and the MS are required before other processes can be performed, they are set in the first part of an HS-SCCH subframe.
The Transport Block Size (TBS) indicates the size of a transport block on a transport channel. The TBS is closely related to the MS and CCS and also to the subject matter of the present invention, which will be described later in great detail.
HARQ introduces two new schemes to increase the transmission efficiency of Automatic Repeat reQuest (ARQ). One of them is a retransmission request from a UE and a response from a Node B, and the other is temporary storage of erroneous data and combining of initial transmission data with retransmission data.
n-channel Stop and Wait (SAW) HARQ was introduced to HSDPA to overcome the shortcomings of a conventional SAW ARQ. The SAW ARQ does not transmit a current data packet until an ACKnowledgement (ACK) is received for the previous data packet. Therefore, even though the current data packet can be transmitted, the ACK for the previous data packet has to be awaited.
In contrast, the n-channel SAW HARQ can increase channel use efficiency by successively transmitting a plurality of data packets although an ACK for the previous packet is not yet received. To be more specific, n logical channels are established between the UE and the Node B and the logical channels are identified by their specific times or channel numbers. Thus, upon receipt of a data packet, the UE can identify the logical channel that has delivered the data packet and take a necessary action such as rearranging data packets in a proper order or soft-combines corresponding data packets. The HARQ Process ID (HAP) in Table 1 indicates a logical channel that delivers a data packet among the n logical channels.
The Redundancy and Constellation Version (RV) varies with a modulation scheme. The RV is given as Table 2 for 16QAM and as Table 3 for QPSK. In Tables 2 and 3, Xrv denotes an RV coding value according to parameters s and r or parameters s, r and b. The parameters s and r are used for rate matching.
TABLE 2Xrvsrb01001000211130114101500261037010
TABLE 3Xrvsr010100211301412502613703
The parameter b in Table 2 is information about constellation rearrangement, set forth in Table 4. Transmission is carried out in one of the following four ways.
TABLE 4Output bitbsequenceOperation0s1, s2, s3, s4None1s1, s2, s3, s4Swapping MSBs with LSBs2s1, s2, s3, s4Inversion of the logical values of LSBs3s3, s4, s1, s21 & 2
The above-described control information bits of the HS-SCCH are dependent on an ACK/Negative ACK (NACK) and a Channel Quality Indicator (CQI) transmitted from the receiver to the transmitter. In the case where the transmitter is to transmit a new packet in response to an ACK received from the receiver, the transmitter notifies the receiver that a new data packet is to be transmitted by the New Data Indicator (NDI). At the same time, the transmitter notifies the receiver of the RV parameter and the HAP. Also, the transmitter determines a modulation scheme and the number of channelization codes according to a CQI received from the receiver and notifies the receiver of the determined modulation scheme and number of channelization codes by the MS and CCS. Consequently, the control information bits of the HS-SCCH are determined based on the ACK/NACK and CQI received from the receiver.
This control information flow between the transmitter and the receiver is illustrated in FIG. 2. Referring to FIG. 2, at an initial transmission, the transmitter sets the NDI to ‘NEW’ to notify the receiver of the initial transmission. The transmitter also notifies the receiver of the parameters s, r and b used for the transmission by the RV coding value, Xrv. Xrv is selected from 0 to 7 illustrated in Table 2 or Table 3, which is expressed as ‘Xrvε{0˜7}’ in FIG. 2.
After decoding a received packet, the receiver determines whether to transmit an ACK or NACK and transmits to the transmitter the ACK or NACK on a High Speed Downlink Dedicated Physical Control CHannel (HS-DPCCH). If receiving the NACK, the transmitter needs to retransmit the transmitted packet. Hence, it sets the NDI to ‘CONTINUE’ and selects one of Xrv values 0 to 7. On the other hand, upon receipt of the ACK, the transmitter sets the NDI to ‘NEW’ and selects one of Xrv values 0 to 7 to transmit a new packet.
Regarding the UE Identification (UE-ID), the UE is assigned up to four HS-SCCHs, as stated earlier, and has to monitor each SCCH subframe to detect an SCCH with its UE-ID. A 16-bit UE-ID is not included as bit information. It is expanded to 40 bits and masked onto Part 1 after rate matching. Therefore, the UE cannot compare its UE-ID with a received UE-ID directly from a decoded bit sequence. It uses the UE-ID as a criterion to determine the reliability of decoding.
FIG. 3 is a block diagram of an HS-SCCH encoder for encoding control information to be transmitted on the HS-SCCH. Referring to FIG. 3, an SCCH information controller 100 generates control information, i.e. Xms, Xccs, Xtbs, Xhap, Xndi, and Xue representing an MS, a CCS, a TBS, an HAP, an NDI, and a UE-ID respectively, and HARQ-related information such as parameters s, r and b with which to generate an RV.
A multiplexer (MUX) 102 multiplexes Xms and Xccs to X1. An RV encoder 110 generates Xrv using the parameters s, r and b. A MUX 112 multiplexes Xtbs, Xhap, Xrv and Xndi to X2.
A channel encoder 104 encodes X1 to Z1. A rate matcher 106 rate-matches Z1, thus outputting R1. The channel encoder 104 uses a rate 1/3 convolutional code. 8-bit Part 1 control information is extended to 40 bits by channel encoding in the channel encoder 104 and rate matching in the rate matcher 106. A UE-specific masker 108 masks R1 with Xue and the resulting S1 is mapped to Part 1 of the HS-SCCH by a physical channel mapper 120.
A UE-specific CRC attacher 114 generates a 16-bit CRC according to the UE-ID for the total sequence of Part 1 and Part 2 (X1+X2) received from the MUXes 102 and 112 and attaches the CRC to Part 2. The resulting Y is encoded in a channel encoder 116 using a rate 1/3 convolutional code. Z2 output from the channel encoder 116 is rate-matched in a rate matcher 118. 13-bit Part 2 control information is extended to 80 bits by CRC addition in the UE-specific CRC attacher 114, channel encoding in the channel encoder 114, and rate matching in the rate matcher 118. The output R2 of the rate matcher 118 is mapped to Part 2 of the HS-SCCH by the physical channel mapper 120.
As described before, the transmitter transmits the control information on the HS-SCCH two slots before data transmission on the HS-PDSCH, to thereby provide information required for demodulation and decoding of the HS-PDSCH. The receiver demodulates the HS-PDSCH based on the control information received on the HS-SCCH.
With reference to FIG. 4, a transport block used for high-speed downlink packet transmission in HSDPA will be described in more detail. As illustrated in FIG. 4, the transport block includes a Media Access Control (MAC)-hs header and MAC-hs payload. The MAC-hs payload has MAC-hs Service Data Units (SDUs) and padding bits. The MAC-hs SDU are equivalent to MAC-d Protocol Data Unit (PDU). The size of the MAC-hs header is variable, and the padding bits are optional in the transport block.
The MAC-hs header includes the following fields:                Version Flag (VF): The VF field is a 1-bit flag providing extension capabilities of the MAC-hs PDU format. The VF field shall be set to be zero and the value one is reserved in the current version of the protocol.        Queue ID: The Queue ID field provides identification of a reordering queue in the receiver, in order to support independent buffer handling of data belonging to different reordering queues. The length of the Queue ID field is 3 bits.        Transmission Sequence Number (TSN): The TSN field provides an identifier for the transmission sequence number on the HS-DSCH. The TSN field is used for reordering purposes to support in-sequence delivery to higher layers. The length of the TSN field is 6 bits.        Size Index Identifier (SID): The SID field identifies the size of a set of consecutive MAC-d PDUs. The MAC-d PDU size for a given SID is configured by higher layers is independent for each Queue ID. The length if the SID field is 3 bits.        Number of MAC-d PDUs (N): The number of consecutive MAC-d PDUs with the same size is identified with the N field. The length of the N field is 7 bits. In Frequency Division Duplex (FDD) mode, the maximum number of PDUs transmitted in a single Transmission Time Interval (TTI) shall be assumed to be 70.        Flag (F): The F field is a flag indicating if more SID fields are present or not. If the F field is set to ‘0’, the field is followed by an additional set of SID, N and F fields. If the field is set to ‘1’, the F field is followed by a MAC-d PDU.        
The padding bits in the MAC-hs payload of the transport block are optionally included to match the transport block size and they do not provide any information. As far as the information bits are concerned, the padding bits are unnecessary.
When the transport block is mapped to a physical channel, the addition of unnecessary the padding bits to the transport block leads to a lower coding rate than without the padding bits. Hence, if the transport block is without the padding bits, the resulting increase in coding rate increases throughput.
In this case, the transport block size is changed by as many bits as the number of the padding bits, and thus different from the TBS known to the receiver. Hence, the size of the changed transport block must be sent to the receiver, with additional information bits.
Considering that the HSDPA channel is transmitted every 2 ms, however, the notification is not easy. It is preferable to inform the receiver of the transport block size in which the number of padding bits is taken into account without additional information bits. Also, since channel encoding is performed on a transport block without the padding bits, the channel decoder of the receiver has to decode a received transport channel correspondingly. That is, the receiver has to perform de-rate matching and channel decoding according to the size of a transport block rate-matched and channel-encoded in the transmitter.